Secondary batteries are used as the distribution of power sources for electronic devices, such as mobile communication equipment, pda notebooks, digital cameras, digital camcorders, electric vehicles (EV), and hybrid electric vehicles (HEV). In particular, high power and energy of the transportation vehicles largely rely upon secondary batteries. In addition, lithium secondary batteries have been drawing attention in terms of there use in high voltage and high energy density applications. Thus, there exists a strong need for high-performance secondary batteries. The characteristics required for such secondary batteries include excellent charge-discharge characteristics, long life characteristics, high-rate characteristics, and good thermal stability at high temperatures.
Lithium secondary batteries are classified into lithium batteries that use lithium metal as a negative electrode and lithium ion batteries that use carbon negative electrodes that are capable of intercalating/deintercalating lithium ions in an inter-layered compound. Lithium secondary batteries are often classified by the type of electrolyte. For example, some type are liquid type batteries, gel type polymer batteries, and solid polymer batteries.
In commercial lithium-ion secondary batteries, LiCoO2 is typically used as the positive electrode material and graphite is used as a negative electrode material. Other positive electrode materials include LiNiO2, LiCoxNi1-xO2, LiMn2O4, and other conventional lithium compounds known in the art. LiCoO2 is stable to charge-discharge events and it exhibits favorable discharge voltage characteristics. However, cobalt is an expensive metal with unfavorable environmental toxicity. Therefore, the use of cobalt in large scale batteries may be prohibitive.
LiNiO2 is one alternative material to LiCoO2. However, Ni-based metal oxides are expensive, primarily due to the cost of Ni, are difficult to synthesize, and possess poor thermal stability. LiMn2O4 spinel is a widely used positive electrode material, due to its relatively low cost and ease of synthesis. However, in spinel-type LiMn2O4 electrodes for 4V grade secondary batteries, Mn ions are easily dissolved, which may lead to poisoning of the graphite anode. Also, the theoretical charge capacity of LiMn2O4 is only about 148 mAh/g, which is lower than the other positive Li-ion electrode materials. Also, its theoretical energy is only about half that of the other positive Li-ion electrode materials.
Methods of preparing the cathode materials include both solid-state and wet methods. Solid-state reactions typically include the mixing and grinding of carbonates or hydroxides of each of the transition metal constituent elements, and then firing the ground mixture. The procedure is typically repeated several times. However, solid-state reaction processes surffer the following drawbacks: i) irregular phases due to non-homogeneous reaction conditions, ii) particle shape and size is difficult to control, and iii) high production temperature and long production time are required. Unlike solid-state reaction processes, wet chemical processes, also known as a sol-gel process, allow from the control of each constituent element at the element level. Using sol-gel methods, high purity transition composite oxides may be obtained. However, such production methods tend to be very expensive. This method uses a starting material such as a soluble acetate (e.g., lithium acetate, cobalt, nickel, manganese acetate) in which the residual water is removed. This requires long production times, and particle agglomeration and uniformity problems are exhibited.
Thus, there is a need for a composite oxide positive electrode material having a layered crystal structure, and methods for making the same, which are capable of solving at least some of the above-referenced shortcomings of present technology, while at the same time, maintaining the advantages of the Co, Ni, and Mn oxides.